The present invention relates to a bias current generating circuit, laser diode driving circuit, and optical communication transmitter.
A circuit for driving a laser diode amplifies a high-speed digital signal output from a time multiplexing circuit called a serializer or multiplexer, and outputs a driving current necessary to drive the laser diode.
This laser diode driving circuit is required not only to amplify a high-speed signal but also to supply a temperature-dependent driving current.
Generally, when the temperature rises, a laser diode increases an emission threshold current and its emission efficiency lowers. The emission threshold current is the value of a driving current for starting light emission. The emission efficiency is the value obtained by differentiating the optical output signal power amplitude by the driving current.
The increase in emission threshold current is suppressed by controlling the current value of a bias current generating circuit installed separately from a high-speed-signal processing amplifier in the laser diode driving circuit. This control is to receive an output current from a monitoring photodiode formed close to the laser diode, and adjust the bias current in accordance with the current value.
A direct current generated by this bias current generator determines the average emission power of the laser diode. The monitoring photodiode senses this average emission power, and the signal is returned to the bias current generator. By this negative feedback path, the average emission power can be controlled independently of the temperature.
A method of compensating for the decrease in emission efficiency caused by the temperature rise of the laser diode will be explained below.
FIG. 5 shows the arrangement of a high-speed-signal amplifier of the laser diode driving circuit. This high-speed-signal amplifier has a driving current controller 1, differential output unit 2, and differential amplifier DA100 as a driving stage.
A pair of differential signals are input to a non-inverting input terminal IN+ and inverting input terminal IN− of the differential amplifier DA100. Through this driver stage, the signals are input to the differential output unit 2 as a final amplification stage and output from it.
The differential output unit 2 is a differential circuit ECL (Emitter Coupled Logic) including bipolar transistors Q200 and Q201. However, the differential output unit 2 can also be constructed using FETs such as MESFETs or MOSFETs, instead of bipolar transistors.
The differential output unit 2 includes resistors R100 and R101, the bipolar transistors Q200 and Q201 making a differential pair, and a bipolar transistor Q202 as a current source transistor.
A bias current to be supplied to the base of the bipolar transistor Q202 is controlled by the driving current controller 1. Although the emitter of the bipolar transistor Q202 is directly grounded in FIG. 5, this emitter may also be grounded via a resistor.
The driving current controller 1 has a bias-current generating circuit BGC1 for generating a bias current Ibias, and bipolar transistors Q100 and Q101, and forms a current mirror circuit together with the transistor Q202 of the differential output unit 2.
The bias current Ibias generated by the bias current generating circuit BGC1 must be preset so as to rise at a desired ratio when the temperature rises, in order to meet the characteristics of the laser diode.
The conventional bias current generating circuit will be described below with reference to FIG. 6.
This bias current generating circuit comprises a bandgap reference circuit BGRC, low-potential-side, constant-current source circuits LCS1 and LCS2, and a current mirror circuit. The bandgap reference circuit BGRC includes resistors R1, R2, R3, and R4, NPN transistors Q1 and Q2, an N-channel transistor N1, and an operational amplifier OP1. The low-potential-side, constant-current source circuit LCS1 includes an N-channel transistor N3, operational amplifier OP4, external terminal PAD1, and external resistor R7. The low-potential-side, constant-current source circuit LCS2 includes an N-channel transistor N4, operational amplifier OP5, external terminal PAD2, and external resistor R9. The current mirror circuit includes P-channel transistors P2 and P3.
The parameters of the resistors R1, R2, R3, and R4, NPN transistors Q1 and Q2, N-channel transistor N1, and operational amplifier OP1 are so set that the circuit including these elements operates as the bandgap reference circuit BGRC.
Accordingly, an output potential V2 from the operational amplifier OP1 maintains about 1.2 V independently of the temperature and a power supply voltage Vcc. In contrast to the potential V2, a contact potential V1 proportional to absolute temperature is generated from the connection node between the resistors R3 and R4. At room temperature, the potential V1 is half (about 0.6 V) the potential V2.
The NPN transistor N1 forms a startup circuit controlled by an activation signal Startup which momentarily changes to high level when the power supply is turned on and then rapidly goes to a ground potential Vss. The NPN transistor N1 allows the bandgap reference circuit BGRC to reach a desired operating point immediately after the power supply is turned on.
Two constant-current source circuits which use the two potentials V1 and V2 generated by the bandgap reference circuit BGRC as reference potentials generate electric currents I1 and I2, respectively.
That is, a first constant-current source circuit including the operational amplifier OP4, NPN transistor N3, and resistor R7 generates the electric current I1 (=V1/R7), and a second constant-current source circuit including the operational amplifier OP5, NPN transistor N4, and resistor R9 generates the electric current I2 (=V2/R9). The resistor R7 is connected between the external terminal PAD1 and ground voltage Vss, and the resistor R9 is connected between the external terminal PAD2 and ground voltage Vss. The resistors R7 and R9 are formed outside a semiconductor integrated circuit forming the laser diode driving circuit, and implemented by fixed resistors, variable resistors, electronic volume ICs, or the like.
The electric currents I1 and I2 are added to form an electric current 13 which functions as a reference current of the current mirror circuit formed by the two PMOS transistors P2 and P3. As a consequence, the bias current Ibias amplified by the gate width ratio (M) of the PMOS transistor P3 to the PMOS transistor P2 is output as a mirror current. This bias current Ibias is the bias current Ibias finally output from the bias current generating circuit BGC1 in the driving current controller 1 shown in FIG. 5. The transistors Q100, Q101, and Q202 form a current mirror. The collector current of the transistor Q202 of the differential output unit 2 is the value obtained by multiplying the size ratio of Q202 to Q101 by the reference current IIbias. Consequently, the laser diode driving current amplitude is proportional to the reference current Ibias.
From the foregoing, letting T denote absolute temperature, Ibias is represented by                                                         Ibias              =                              M                ×                I                ⁢                                                                  ⁢                3                                                                                        =                              M                ×                                  (                                                            I                      ⁢                                                                                          ⁢                      1                                        +                                          I                      ⁢                                                                                          ⁢                      2                                                        )                                                                                                        =                              M                ×                                  {                                                            (                                              V                        ⁢                                                                                                  ⁢                                                  1                          /                          R                                                ⁢                                                                                                  ⁢                        7                                            )                                        +                                          (                                              V                        ⁢                                                                                                  ⁢                                                  2                          /                          R                                                ⁢                                                                                                  ⁢                        9                                            )                                                        }                                                                                                        =                              M                ×                                  {                                                            A                      ×                      T                                        +                    B                                    }                                                                                        (        1        )            where A and B are constants and represented byA≅(0.002/R7)×T  (2)B≅1.2/R9  (3)
FIG. 7 shows an example of the temperature dependence of each of the electric currents I1, I2, and I3.
The ratio of the electric current I1 to the electric current I2 can be changed by the values of the resistors R7 and R9. When the ratio of the electric current 12 is raised, the temperature dependence of the bias current Ibias decreases. When the ratio of the electric current I1 is raised, the temperature dependence of the bias current Ibias increases.
As described above, by adjusting the values of the external resistors R7 and R9 in accordance with the temperature dependence of the emission efficiency of each individual laser diode, the optical output amplitude of the laser diode can be held constant regardless of the temperature.
The bias current Ibias of the bias current generating circuit shown in FIG. 6 becomes zero at absolute zero, when the resistor R9 is made infinite, i.e., when the resistor R9 is removed. That is, this bias current generating circuit has characteristics proportional to the temperature.
If the bias current Ibias at a certain temperature To is regarded as a reference, the rate of increase of the bias current Ibias per degree of the temperature is 1/To. If the temperature To is room temperature (300K), the rate of change of the bias current Ibias to the temperature is 1/300≅3333PPM.
Generally, the temperature dependence of the emission efficiency of a laser diode is larger than 3333PPM. The laser diode driving circuit having the bias current generating circuit shown in FIG. 6 cannot perform temperature compensation for such a laser diode. Accordingly, no optical signal power amplitude independent of the temperature can be obtained.
The following is a reference disclosing the conventional current control technique.
Japanese Patent Laid-Open No. 2000-244250.
As described above, the conventional bias current generating circuit cannot well perform temperature compensation for the temperature dependence of the emission efficiency of a laser diode.